1. Field of the Invention
The present invention relates to a noise absorbing material and a method of producing the same. More particularly, the present invention relates to a fire retardant noise absorbing material used in an engine room of a construction machine, an automobile or the like, with excellent waterproof properties, fire resistance, noise absorption, and improved tear resistance if necessary, and a method of producing the same with a small energy loss in a short time in a simple manner.
2. Description of the Related Art
Noise absorbing materials used for engine rooms of construction machines, automobiles, common outdoor articles, and the like require at least an improvement in waterproof properties, noise absorption in a low to medium frequency range, and fire resistance and there is a strong demand for a method of producing such noise absorbing materials with a small energy loss in a short time in a simple manner. While a gas permeability is conferred to a noise absorbing material in order to improve the noise absorption, they are permeated with water thereinto by rainfall and washing, and such water is hard to drain and is often accumulated therein, so that the noise absorption is reduced, it is faster for the material to deteriorate as well and as a result, durability thereof is reduced. Accordingly, there is a need for an improvement in waterproof properties. Fire resistance of noise absorbing materials can also be thought of as an indispensable property from the view point of the safety of a product using noise absorbing materials, regulations by law and the like. While fire retardant soft urethane foam is well known, it cannot satisfy the requirements of waterproof properties and noise absorption in a low to medium frequency range though it has sufficient fire resistance in cases where urethane foam is used alone. As a method for improving noise absorption in the range of low to medium frequency, there are known methods such as that disclosed in Japanese Patent Application Laid-Open (JP-A) No. 61-53035. This method is to improve noise absorption by a stacking plastic film with gas permeability using a hot melting technique on a common soft urethane. However, it is hard to obtain a fire retardant material with such a combination of flammable plastic film and flammable soft urethane. There is a common understanding that fire resistance is reduced in a combination of a flammable plastic film and fire-retardant soft urethane foam. Further, production of a noise absorbing material with specific gas permeability by hot melting is accompanied with various problems. For example, when a hot melt film is simply superimposed on a soft urethane foam and heat is applied on the film by a hot plate, the film sticks to the hot plate and thus affixing of the film on the urethane foam cannot be realized if the plate is separated from the urethane foam as soon as the hot melting of the film finishes. This phenomenon is caused by moving the hot plate, for example, upwardly or downwardly while the film is still in a molten condition. To cope with such problems, it is necessary to separate the noise absorbing material from the hot plate after the hot plate has cooled, that is after the film has solidified. Cooling after heating is a primitive method and to repeat heating and cooling of the heat plate is attended with great energy loss, inefficiency and time consumption.
As mentioned above, at the current level in conventional technology there is not known a method of producing a noise absorbing material which is excellent in properties of water proof, fire resistance, high noise absorption in a low to medium frequency range, and improved on tear resistance if necessary, with a small energy loss, in a short time in a simple manner.